Because trees are tall, not high.
High is used for talking about things that are a long way from the ground, or about things whose top parts are a long way from the ground: a high shelf ♦ a high window ♦ the world's highest mountain
Tall is used about people or things that measure more than is usual from their bottom to their top, especially things that are more high than wide, like a person or a tree: a tall lamp-post ♦ a tall thin bottle ♦ the tallest boy in the class
Why does "jsou to vysoke stromy" not translate to "are they the tall trees"? Isn't it when the demonstrative adjective is on a noun, its marked as definite?
My thinking: "To" is a pronoun here, not a modifier. (If it were a modifier, it would have to be "ty," to agree in gender, number and case with "stromy.")
"To" can mean either "they" or "those" when used in the "To jsou" construction. So the translation could be either "Are THEY tall trees" or "Are THOSE tall trees."
But it can't be "Are THEY/THOSE THE tall trees," because there is no second pronoun which standing in for "the."
Use: "Are those tall trees?"
You may think it is the same but syntactically it is not. And we do distinguish here and in many other sentences.
tall trees are ... / vysoké tromy jsou ...
... trees are tall / ... stromy jsou vysoké
We consider this difference important for lerning the difference in Czech.
Isn't "Strom" masculine. I have it in my earlier notes that it is. But "vysoke stromy" follows (from what I've been able to gather) the feminine pattern for plural (stare zeny). I'm struggling to come up with a pattern I can understand (and therefore apply). Do some nouns change sex in the plural? Isn't Ulice neuter in the singular but feminine in the plural?
Strom is indeed masculine and it is inanimate. Here we have stromy, which is plural and still masculine. Vysoké is used for both feminine and masculine inanimate nouns in the nominative plural. You can see the full declension table here: https://en.wiktionary.org/wiki/vysok%C3%BD. (There are some -- not many -- nouns that do change gender in the plural, but this isn't one of them.)